The invention relates to a flat-track unit for motor vehicle test beds comprising a roadway-simulating belt which is guided over two rollers and accommodates a vehicle wheel on the upper side and which upper side is orientated roughly horizontally at ground level. The belt is supported in a sliding or rolling manner at the wheel contact region by a supporting shoe. The two rollers are mounted on a frame which in supported in a vertically floating manner as it is mechanically guided in the vertical direction to develop vertical oscillations by a vertically orientated hydraulic cylinder (oscillation exciter) arranged in horizontal projection, roughly in the same position as the wheel contact area. One of the two rollers is connected via a universal-joint and shaft to a drive and braking unit, which is arranged to be vertically immovable and spaced a short distance from the frame. This unit has an output shaft which, on a chronological average, is roughly on the same axis as the one driven roller.
Specification U.S. Pat. No. 3,520,180 shows this general type of test bed.
Flat-track units of this general type are intended to reproduce the rolling relationships of the vehicle wheel on the road in as true to nature manner as is possible on the test bed. In the process, the vehicle wheel stands on a flat contact surface which can be driven, or even braked, to simulate braking or traction forces. In addition, roadway vibrations are simulated by vertical oscillations being imparted to the flat-track unit. Since the belt cannot itself provide a flat contact surface on account of its flexibility, the belt, in the contact region of the vehicle wheel, is supported with a supportive shoe which is flat at the top. The supportive shoe is excited by a vertically disposed excitation cylinder to simulate roadway vibrations. In addition, or instead, the frame supporting the rollers can also be excited independently, or in a superimposed manner, to develop vertical oscillations. In order that the frame can be of particularly light construction, the roller drive is arranged in a fixed position physically separated from the frame, with the separation gap being bridged via a movable universal-joint shaft. It is disadvantageous if one cannot run tests in which a significant transverse force can build up between the roadway-simulating belt and the vehicle wheel.
The object of the invention is to further develop the flat-track unit so that it can also reproduce driving conditions in which a considerable transverse force is built-up between the roadway-simulating belt and the vehicle wheel.
This object is achieved by having the oscillation exciter of the frame comprise a cylinder housing which in firmly secured in position at a foundation in such a way as to resist tilting and rotation. A piston rod in the cylinder is designed as a guide column and is rigidly connected to the frame in terms of rotation and tilting and thus is able to serve as a mechanical vertical guide for the frame. The rod is rotatable and together with the cylinder housing forms a pivot bearing for pivoting the frame about a vertical axis. An oscillation exciter is provided with a pivot drive for pivoting the piston rod and hence the frame about the vertical axis. A universal-joint shaft is connected between the drivable roller and the drive and braking unit, and is provided with length-compensation means having a diameter corresponding to a multiple of the diameter of the universal-joint shaft. The drive and braking unit is attached to a drivable slide which is displaceable on a circular arc shaped horizontal guide running concentrically to the vertical axis. The slide drive is movable as a function of the deflection angle of the universal-joint shaft and is displaced along the circular arc in such direction, as to carry the universal-joint shaft into a straightened position, wherein the drive shaft is at right angles to the axis of the driven roller.
The oscillation exciter used for the vertical excitation also serves as a pivot bearing for mounting the frame about a vertical axis. By means of a separate pivot drive, the frame and the roadway-simulating belt can be angled relative to the wheel plane of the vehicle wheel and this angulation can be subjected to a simulated oscillation which is close to reality. In order that the universal-joint shaft does not need to be angled too sharply relative to the associated braking or drive unit, even at larger angular deviations between belt running direction and wheel plane and under corresponding vibrations, the drive and braking unit is transversible on a circular arc and is provided with a corresponding follow-up drive. This follow-up drive constantly keeps the universal-joint shaft in its approximate straightened position. This design requires that only high-frequency inclination oscillations of relatively small deflection have to be absorbed by the universal-joint shaft.
It is advantageous if the piston rod of the oscillations exciter extend axially on both sides of the piston and is guided over the entire longitudinal extent in the cylinder housing of the oscillation exciter.
It is further desirable if the frame is designed in such a way that its supporting parts are essentially restricted to the area between the bearings of the two belt guiding rollers and the frame mass is concentrated as close as possible to the area around the vertical axis. This is accomplished by having the frame formed by two side cheeks, which are arranged on both sides of the belt to support the bearings for the rollers, and which are connected in the region of the lower belt side by means of two cross-pieces. The two side cheeks (viewed parallel to the roller axes) are, in rough approximation, designed in a trapezoidal shape, or in an extended V-shape.
It is also advantageous if the piston rod is of hollow design and is attached inside the hollow of the piston rod. The pivot drive is designed as a pivoted-piston drive and its output shaft is arranged on the same axis as the piston rod and connected to it so as to be fixed in terms of rotation. The output shaft is also provided with a length-compensation means for compensating vertical changes in the piston rod on account of the excitation or oscillation in the vertical direction.
It is desirable that the circular arc-shaped horizontal guide accommodating the slide extend over such an angular range that (starting from a normal position of the universal-joint shaft orientated transversely to the test-bed longitudinal axis) the slide, by virtue of its contact length on the horizontal guide, is pivotable by .+-.50.degree..
Preferably the drive for displacing the slide on the horizontal guide is designed in the form of a rectilinear displacement cylinder with pistons which can be acted upon on both sides. The displacement cylinder, in the normal position of the slide, is orientated roughly tangential to the horizontal guide. The piston rod of the displacement cylinder is designed to be hollow and has a hydraulic cylinder on the inside for accommodating a piston held therein in a fixed position via a piston rod, and wherein the two effective piston areas, located opposite one another, are approximately the same size.
It is further advantageous if the drive for displacing the slide on the horizontal guide is effected by a pinion which can be driven by a hydrostatic motor mounted on the slide for meshing with an arc-shaped toothed rack that follows the horizontal guide.
It is preferred that the length-compensation means for the universal-joint shaft (between the driving roller and the drive or braking unit) is arranged inside the hollow shaft (push-in journal) of the drive and braking unit. The push-in journal is designed as a four-cornered flat sided journal, square in cross-section, and the hollow shaft has a corresponding four-cornered driver in its interior. The four-cornered driver of the hollow shaft has at least four, preferably eight, linear antifriction roller guides having roller circulation in the form of antifriction roller guide shoes, each of which is supported in the wall of the four-cornered driver. Antifriction roller guide shoes are allocated to each flat side of the four-cornered journal with their antifriction rollers bearing under pretension against the flat sides of the four-cornered journal. The hollow shaft having the four-cornered driver is formed by a cylindrical opening in a machine part containing the hollow shaft and into which a bush, cylindrical on the outside and having the four-cornered driver on the inside, can be inserted free from play and can be screwed to the machine part so as to be fixed in terms of rotation. The bush is split along a transverse plane running through the rotational axis and through two opposite edges of the four-cornered journal and is screwed together by tangentially running screws. The bush includes spacer strips, each ground individually to the thickness necessary for the desired antifriction roller pretension and being interposed in the region of the parting surfaces of the bush. The transversely split bush (after it has been assembled with the desired roller pretension, together with the four-cornered journal accommodated in it) is turned on its outside diameter to obtain the desired cross-section to be secured within the cylindrical bush.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.